The purpose of the bone mineral density in childhood study (BMDCS) is to obtain standard reference bone mineral density measures taking into consideration skeletal maturation, bone size and volume, body size, and pubertal stage. These data will serve as an extremely valuable standard pediatric BMD database. It is anticipated to become the essential reference that will enable pediatricians to study, diagnose, and treat children with disorders affecting bone accrual. The six contracts for the Clinical Centers and the Data Coordinating Center were awarded on August 1, 2001. The five academic clinical centers all adhere to a common longitudinal observational protocol involving a baseline and three consecutive annual evaluations, including dual x-ray absorptiometry (DXA) and bone age x-ray, a physical exam to determine their pubertal status, stadiometer height, weight, and body mass index (BMI). Additionally, three sites are performing CT or pQCT scans on a subset of children. Information relating to bone health such as diet and exercise is obtained from each child annually. The bone age x-ray and DXA scans are interpreted centrally. Recruitment commenced in July 2002 at five Clinical Centers and resulted in the enrollment by November 1, 2003 of 1554 children from 6-16 years old of both genders and all racial groups.